In a previously reported study, it was found that the fatigue crack growth rate actually accelerated after a tensile overload under a negative value of baseline stress ratio R. This type of crack propagation behavior is related to the change in the compression residual stress to tensile residual stress distributed in the vicinity of the crack tip upon unloading to minimum load after an overload. In the present investigation, focused on the negative stress ratio, it was found that the deceleration and acceleration of crack propagation were associated with crack opening displacement at the overload point and the conditions of applied baseline cyclic stress. The fatigue crack growth rate decelerated after being overloaded at a positive R value. However, that rate accelerated rather than decelerated in some stress conditions at a negative R value. Therefore, the residual fatigue life after overloading became longer or shorter, depending on the overload level and baseline stress conditions.
The mechanism of crack growth of precracked materials was investigated under cyclic torsion loading with and without axial static stress using 4340 steel. During the crack growth, branching of the crack was observed. The length of the crack between the first branching points was dependent on loading conditions. This length was longer when the applied shear amplitude and the static axial stress level were higher. When the crack tips were opened by the tension loads, the crack had the tendency to grow in a shear mode during cyclic torsion. It was found that friction of the crack surfaces prevented shear mode crack growth. Furthermore, at higher stresses the initiation of new microcracks was observed in front of the main crack and their density was dependent on loading conditions. This helped the crack grow in a shear mode before and after branching.
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